This invention relates to a doctoring apparatus of the type employing an elongated flexible coiled blade, and is concerned in particular with an improved mechanism for traversing the blade material across the surface being doctored.
In the traditional doctoring apparatus, the blade is normally of a length approximately equal to the face length of the surface being doctored, for example the cylindrical surface of a rotating cylinder. In order to more evenly distribute wear of the cylinder surface and blade working edge, the blade support structure is often arranged to reciprocate to and fro through a relatively short stroke of approximately one inch or less. Eventually, however, when the blade becomes worn, it must be removed and replaced with a fresh blade. This unavoidably results in an interruption of the production process, which in turn increases product costs while at the same time adversely affecting product quality.
In recent years, attempts have been made at reducing lost production time occasioned by the need to make blade changes. These attempts have focused on feeding coiled blade stock across the width of the surface being doctored. A typical example of this approach is described in U.S. Pat. No. 4,528,067. Experience has indicated, however, that such arrangements have serious limitations because of high induced stress levels in the blade material and also because of the high torque levels required to drive the take up reels.
For example, during a normal doctoring operation, depending on the length of the blade in contact with the surface being doctored and the force with which the blade is being applied to that surface, the forces required to pull the blade through the blade holder can range between 2000-6000 pounds. Factors such as contaminants in the blade groove and high friction along the blade working edge can neccessitate even higher blade pulling forces.
In addition to the tension stresses produced by these pulling forces, the blade material also experiences bending stresses as the blade stock is wound onto the take up reels. Thus, where a drive reel is used to pull the blade stock through the holder, the combination of tensile, bending and radial stresses acting on the blade cross section can exceed the yield strength of the blade material, thereby causing breakage.
It has been recognized that the above noted bending stresses are inversely proportional to the radius of curvature that the blade stock is forced to take as it is wound onto the drive reel. For example, with a 0.050" thick blade being pulled through the holder with a 3000 pound force, a radius of curvature at the reel of 6-1/4" produces a bending stress of approximately 150,000 p.s.i., and necessitates a reel driving torque of 18,750 lb.-in. If the radius of curvature at the reel is increased to 15", bending stresses are beneficially reduced to approximately 63,000 p.s.i., but the reel driving torque is increased dramatically to 45,000 lb.-in. These high driving torques in turn necessitate large and expensive drive mechanisms.
Thus, it will be seen that with conventional arrangements employing driven reels to pull the blade stock through the holder, small diameters result in unacceptably high bending stresses with relatively low driving torques. Conversely, large diameters lower the bending stresses to acceptable levels, but the driving torques are boosted to levels which require unacceptably large and expensive drives.
A basic objective of the present invention is to provide an improved mechanism for longitudinally moving the blade stock across the surface being doctored, without generating high bending stresses, and without having to employ large and expensive drive mechanisms.